Biometric sensors, and in particular fingerprint sensors, are well known today. Such sensors are an element of a variety of different devices which use identification and matching of a unique biometric attribute (e.g., a fingerprint pattern) to control access to buildings, computers, safes, software, etc. For the purposes hereof, the focus is on fingerprint sensors within the class of biometric sensors, although the background and embodiments described herein may be equally applicable to sensors for other unique biometric attributes of a user.
Typical fingerprint sensors today comprise a semiconductor structure on the surface of which is formed an array of sensor elements, and optionally circuitry for driving the sensor array and manipulating the signals issuing therefrom. Typical fingerprint sensing systems include such sensors housed in a body for mounting into laptop computers, cell phones, door locks, etc.
One common form of sensor used today for fingerprint sensing is a so-called capacitive sensor. These devices operate by capacitive electric field sensing or by generating a fringing electric field between capacitive plates in a sensor cell. The volume of material entering the fringing field changes the fringing field for that cell. That change can be measured and correlated to the ridges and valleys of a fingerprint proximate that cell. There are two common types of capacitive sensors, the area sensor and the strip sensor. In an area sensor, a user places a finger over the sensor array, and the entire fingerprint is read from the array with the finger in place. In a strip sensor, a user swipes a finger over a narrow array of sensors. The motion of the finger is correlated with the data from the sensor, and a digitized image of the fingerprint is assembled by software. In each type of sensor, the sensitivity of the sensor is a function of the proximity of the fingerprint to the plates of the capacitor. As the distance between the finger and the sensor increases the electric field strength decreases, and the strength of the effect of the presence of tissue in the field decreases. Furthermore, should conductive material be interposed between the finger and the sensor, the fringing field will be affected, and the sensing accuracy will deteriorate.
Typically, the sensors can tolerate only a minimal gap between the sensor surface and the fingerprint to be sensed. For example, one type of sensor, referred to as a capacitive sensor, uses the effect of the relative spacing of the ridges and valleys of a fingerprint within a capacitive electric field to digitize the fingerprint pattern. Therefore, the sensor surface itself is commonly left uncovered or thinly covered, and a user places a finger directly into contact therewith in the process of fingerprint sensing. However, an exposed or thinly covered sensor is susceptible to contamination from the environment and mechanical damage.
U.S. Pat. No. 6,376,393 assigned to the assignee of the present invention and incorporated herein by reference in its entirety, discloses an anisotropic coating for a fingerprint sensor that is fabricated by applying a magnetic field to a solidifiable dielectric fluid, and then solidifying the fluid. This produces an impedance perpendicular to the anisotropic dielectric layer that is less than an impedance parallel to the layer. This patent also discloses another type of electric field-based sensing pixel that is driven by an RF signal.
Furthermore, the assembly of a fingerprint sensor typically includes a die on which the sensor array is formed. The die is secured to a substrate, which may itself include processing electronics to process the signals provided by the sensor array. The sensor array is often therefore electrically interconnected to the substrate, for example, by way of wire bonds which connect to bonding pads on the top surface of the die, loop up and over the edge of the die, and ultimately connect to bonding pads on the substrate. These wire bonds are critical but fragile elements that are typically protected by encasing them in non-conductive encapsulation material. Due to the need to minimize the gap between the sensor array and the user's finger, the encapsulation material is molded in such a way that the wire bonds are adequately encapsulated, yet the sensor surface is uncovered or thinly covered. Since one end of the wire bonds attaches to the top surface of the sensor die, the wire bonds typically extend to a height above the surface of the sensor die. This means the top surface of the molded device includes a first region of the sensor which is in a first plane, and a second region above the wire bonds which is in a second plane above the first plane. However, molding to form such thin encapsulation covering may be relatively complex and costly.
In addition, molding materials are well known and well established. There is therefore a strong desire to utilize existing molding material. However, existing molding materials have dielectric properties that may make them less than optimal when used with electric field-based devices. Thus, efforts are typically taken to minimize the thickness of the molding material over the sensor, for the reasons discussed above.
Furthermore, when using the encapsulation material for a sensor covering, the only option for the color of the covering is the color of the encapsulation material. This may not match the design requirements for the sensor device, as may, for example, be dictated by customer requirements, branding preferences, etc.
Still further, there may be a desire to provide illumination associated with the sensing function. Light sources associated with sensor packages may provide a user with some sort of visual feedback during operation of a device. An example is disclosed in U.S. Pat. No. 7,272,723, incorporated herein by reference in its entirety, in which a light emitting diode (LED) provides a user with a visual indication of the operations being performed by a peripheral key management device. However, there may be a desired to cover the light sources in such devices both to protect the light sources and to improve the device aesthetics.
In such known devices, the light source may be in electrical communication with a substrate which has electrical connection to processing hardware, or is in electrical communication with other driving means. The light source and substrate may optionally be enclosed within the interior of a partially translucent or transparent housing. In such an embodiment, the housing is a structure molded prior to introducing the substrate and LED. During assembly of the final device, the substrate and LED are brought together and secured within the pre-molded housing. While electrically coupled to the substrate, the light source is not mounted on or an integral part thereof. Thus, such separate housing, substrate, and LED assemblies are relatively large, limiting the types of devices into which such assemblies may be integrated. In addition, opportunities to reduce cost and simplify manufacture remain.